As the demand for longer transmission distances and higher information transfer rates for optical fibre communication increases, so does the importance of measuring the state of polarization of light within an optical fibre. The transfer properties of some optical devices such as polarization dependent isolators, couplers and optical amplifiers depend on the polarization state of light launched into them. To completely characterize these devices, the relationship of the input and output states of polarization (SOP) of the optical system must be known.
One known way of measuring the SOP of a light beam is to align a waveplate and a linear polarizer in the optical path of the beam. The waveplate is rotatable about the optical axis and typically is a quarter-wave plate. An optical sensor, a photodetector, is positioned to measure the intensity of light transmitted by the waveplate and polarizer. In operation the waveplate is sequentially rotated to a minimum of four angular positions about the optical axis relative to the linear polarizer and the transmitted light intensity is measured at each position by the photodetector. A disadvantage of this method is the mechanical movement of the waveplate and the resulting slow speed of measurement. Additionally, since every optical element has to align in free space, miniaturization of such a device in not possible.
Several polarimeters are described in U.S. Pat. No. 5,815,270 in the name of Lee, entitled IN-LINE FIBER-OPTIC POLARIMETER USING A FUSED 1.times.5 STAR COUPLER. Lee's polarimeter shown in prior art FIG. 1, and other prior art polarimeters as well, depend on splitting a beam of light through a splitter and passing the sub-beams into three polarizing fibres. In FIG. 1 a single beam launched into a 1.times.5 star coupler provides sub-beams to three polarizers 22, 38, and 40. In Lee's disclosure, polarizing fibres are utilized. Although Lee's device performs its intended function, this and other devices based on using a plurality of polarizers are not particularly efficient since only a fraction of the light in each of three beams being launched into respective polarizers 22, 38, and 40 in Lee's device is utilized. Stated differently, each detector at the output of a polarizer is only detecting a fraction of the light launched into a polarizer, and hence, a considerable amount of energy is wasted.
It is an object of this invention to provide an optical circuit and method for determining the SOP which conserves energy, wherein most of the input light is directed to the detectors.
In contrast to the prior art devices, the method and circuit does not "spill or reject" much of the light launched into the polarimeter. Most of the light under test in accordance with this invention is utilized, and advantageously, less light is required.